Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Tens of MeV proton beams have been generated by interactions of the VULCAN petawatt laser with foil targets and used to induce nuclear reactions in zinc and boron samples. The numbers of ¹¹C, ⁶⁶Ga, ⁶⁷Ga, ⁶⁸Ga, ⁶¹Cu, ⁶²Zn, ⁶³Zn, and ^69mZn nuclei have been measured and used to determine the proton energy spectrum. It is known that (p,n) reactions provide an important method for producing neutron sources and in the present experiment up to ∼10⁹ neutrons sr^-1 have been generated via ¹¹B(p,n)¹¹C reactions. Using experimentally determined proton energy spectra, the production of neutrons via (p,n) reactions in various targets has been simulated, to quantify neutron pulse intensities and energy spectra. It has been shown that as high as 4 × 10⁹ neutrons sr^-1 per laser pulse can be generated via ⁷Li(p,n) ⁷B reactions using the present VULCAN petawatt laser-pulse conditions. © 2004 American Institute of Physics.

Abstract:

We present the observation of a nonlocal heat wave by measuring spatially and temporally resolved electron temperature profiles in a laser produced nitrogen plasma. Absolutely calibrated measurements have been performed by Rayleigh scattering and by resolving the ion-acoustic wave spectra across the plasma volume with Thomson scattering. We find that the experimental electron temperature profiles disagree with flux-limited models, but are consistent with transport models that account for the nonlocal effects in heat conduction by fest electrons.

Abstract:

Petawatt (PW) lasers are unique tools to study plasmas under extreme conditions. There are many applications for these plasmas that potentially have an impact on a wide range of scientific disciplines. A number of these are highlighted here in this review including: fast ignition of fusion targets; high brightness x-ray harmonic generation from oscillating plasma surfaces and the production of super-strong magnetic fields. This is a rich field of investigation, and space prevents a detailed discussion of some of these fascinating topics, including electron and ion acceleration processes that were highlighted at the London conference. Fortunately, they are presented elsewhere in other invited papers in this special issue.

Abstract:

Recent progress in the physics of fast ignition of fusion targets is reviewed here. Fundamental studies on hot electron energy transport show that the scheme looks promising if the heating pulse can be guided close enough to a compressed core. The idea of using cone-guided compression was first demonstrated experimentally under a Japan-UK collaboration. The use of the gold cone was extremely successful and showed a 10³ neutron increase out of CD target implosion with a 300 J/0.5 ps enforced heating laser pulse. The heated temperature was close to 1 keV. In order to increase the temperature to 10keV, a 10kJPW^-1 laser system is necessary. Osaka University has started constructing such a laser system.

Abstract:

The first experiments on the National Ignition Facility (NIF) have employed the first four beams to measure propagation and laser backscattering losses in large ignition-size plasmas. Gas-filled targets between 2 and 7 mm length have been heated from one side by overlapping the focal spots of the four beams from one quad operated at 351 nm (3ω) with a total intensity of 2 × 1015 Wcm-2. The targets were filled with 1 atm of CO 2 producing up to 7 mm long homogeneously heated plasmas with densities of ne = 6 × 1020 cm-3 and temperatures of Te = 2 keV. The high energy in an NIF quad of beams of 16 kJ, illuminating the target from one direction, creates unique conditions for the study of laser-plasma interactions at scale lengths not previously accessible. The propagation through the large-scale plasma was measured with a gated x-ray imager that was filtered for 3.5 keV x-rays. These data indicate that the beams interact with the full length of this ignition-scale plasma during the last ∼1 ns of the experiment. During that time, the full aperture measurements of the stimulated Brillouin scattering and stimulated Raman scattering show scattering into the four focusing lenses of 3% for the smallest length (∼2 mm), increasing to 10-12% for ∼7mm. These results demonstrate the NIF experimental capabilities and further provide a benchmark for three-dimensional modelling of the laser-plasma interactions at ignition-size scale lengths.